In the course of the following description, reference will be made to the papers, patents and publications presented in a list of references at the conclusion of this specification. When cited, each listed reference will be identified by a numeral within curly-braces indicating its position within this list.
As noted in {1} {2} {3} {4}, an artificial limb system that mimics a biological limb ideally needs to fulfill a diverse set of requirements. The artificial system must be a reasonable weight and have a natural morphological shape, but still have an operational time between refueling or battery recharges of at least one full day. The system must also be capable of varying its position, impedance, and motive power in a comparable manner to that of a normal, healthy biological limb. Still further, the system must be adaptive, changing its characteristics given such environmental disturbances as walking speed and terrain variation. The embodiments of the invention which are described in this specification employ novel actuator and limb architectures capable of achieving these many requirements.
From recent biomechanical studies {1} {2} {3}, researchers have determined that biological joints have a number of features. Among these are:                (a) The ability to vary stiffness and damping.        (b) The ability to generate large amounts of both positive and negative mechanical work.        (c) The ability to produce large amounts of nonconservative power and torque when needed.        
An example of the use of more than one control strategy in a single biological joint is the ankle {1} {2}. For level ground ambulation, the ankle behaves as a variable stiffness device during the early to midstance period, storing and releasing impact energies. Throughout terminal stance, the ankle acts as a torque source to power the body forward. In distinction, the ankle varies damping rather than stiffness during the early stance period of stair descent. These biomechanical findings suggest that in order to mimic the actual behavior of a human joint or joints, stiffness, damping, and nonconservative, motive power must be actively controlled in the context of an efficient, high cycle-life, quiet and cosmetic biomimetic limb system, be it for a prosthetic or orthotic device. This is also the case for a biomimetic robotic limb since it will need to satisfy the same mechanical and physical laws as its biological counterpart, and will benefit from the same techniques for power and weight savings.
In the discussion immediately below, the biomechanical properties of three human joints, the ankle, knee and hip, will be described in some detail to explain the insights that have guided the design and development of the specific embodiments of the invention and to define selected terms that will be used in this specification.
Joint Biomechanics: The Human Ankle
Understanding normal walking biomechanics provides the basis for the design and development of the artificial ankle joint and ankle-foot structures that embody the invention. Specifically, the function of human ankle under sagittal plane rotation is described below for different locomotor conditions including level-ground walking and stair/slope ascent and descent. From these biomechanical descriptions, the justifications for key mechanical components and configurations of the artificial ankle structures and functions embodying the invention may be better understood.
Level-Ground Walking
A level-ground walking gait cycle is typically defined as beginning with the heel strike of one foot and ending at the next heel strike of the same foot {8}. The main subdivisions of the gait cycle are the stance phase (about 60% of the cycle) and the subsequent swing phase (about 40% of the cycle) as shown in FIG. 1. The swing phase represents the portion of the gait cycle when the foot is off the ground. The stance phase begins at heel-strike when the heel touches the floor and ends at toe-off when the same foot rises from the ground surface. Additionally, we can further divide the stance phase into three sub-phases: Controlled Plantar flexion (CP), Controlled Dorsiflexion (CD), and Powered Plantar flexion (PP).
Each phase and the corresponding ankle functions which occur when walking on level ground are illustrated in FIG. 1. The subdivisions of the stance phase of walking, in order from first to last, are: the Controlled Plantar flexion (CP) phase, the Controlled Dorsiflexion (CD) phase, and the Powered Plantar flexion (PP) phase.
CP begins at heel-strike illustrated at 103 and ends at foot-flat at 105. Simply speaking, CP describes the process by which the heel and forefoot initially make contact with the ground. In {1} {3}, researchers showed that CP ankle joint behavior was consistent with a linear spring response where joint torque is proportional to joint position. The spring behavior is, however, variable; joint stiffness is continuously modulated by the body from step to step.
After the CP period, the CD phase continues until the ankle reaches a state of maximum dorsiflexion and begins powered plantarflexion PP as illustrated at 107. Ankle torque versus position during the CD period can often be described as a nonlinear spring where stiffness increases with increasing ankle position. The main function of the ankle during CD is to store the elastic energy necessary to propel the body upwards and forwards during the PP phase {9} {3}.
The PP phase begins after CD and ends at the instant of toe-off illustrated at 109. During PP, the ankle can be modeled as a catapult in series or in parallel with the CD spring or springs. Here the catapult component includes a motor that does work on a series spring during the latter half of the CD phase and/or during the first half of the PP phase. The catapult energy is then released along with the spring energy stored during the CD phase to achieve the high plantar flexion power during late stance. This catapult behavior is necessary because the work generated during PP is more than the negative work absorbed during the CP and CD phases for moderate to fast walking speeds {1} {2} {3} {9}.
During the swing phase, the final 40% of the gait cycle, which extends from toe-off at 109 until the next heel strike at 113, the foot is lifted off the ground.
Stair Ascent and Descent
Because the kinematic and kinetic patterns at the ankle during stair ascent/descent are significantly different from that of level-ground walking {2}, a separate description of the ankle-foot biomechanics is presented in FIGS. 2 and 3.
FIG. 2 shows the human ankle biomechanics during stair ascent. The first phase of stair ascent is called Controlled Dorsiflexion 1 (CD 1), which begins with foot strike in a dorsiflexed position seen at 201 and continues to dorsiflex until the heel contacts the step surface at 203. In this phase, the ankle can be modeled as a linear spring.
The second phase is Powered Plantar flexion 1 (PP 1), which begins at the instant of foot flat (when the ankle reaches its maximum dorsiflexion at 203) and ends when dorsiflexion begins once again at 205. The human ankle behaves as a torque actuator to provide extra energy to support the body weight.
The third phase is Controlled Dorsiflexion 2 (CD 2), in which the ankle dorsiflexes until heel-off at 207. For the CD 2 phase, the ankle can be modeled as a linear spring.
The fourth and final phase is Powered Plantar flexion 2 (PP 2) which begins at heel-off 207 and continues as the foot pushes off the step, acting as a torque actuator in parallel with the CD 2 spring to propel the body upwards and forwards, and ends when the toe leaves the surface at 209 to being the swing phase that ends at 213.
FIG. 3 shows the human ankle-foot biomechanics for stair descent. The stance phase of stair descent is divided into three sub-phases: Controlled Dorsiflexion 1 (CD1), Controlled Dorsiflexion 2 (CD2), and Powered Plantar flexion (PP).
CD1 begins at foot strike illustrated at 303 and ends at foot-flat 305. In this phase, the human ankle can be modeled as a variable damper. In CD2, the ankle continues to dorsiflex forward until it reaches a maximum dorsiflexion posture seen at 307. Here the ankle acts as a linear spring, storing energy throughout CD2. During PP, which begins at 307, the ankle plantar flexes until the foot lifts from the step at 309. In this final PP phase, the ankle releases stored CD2 energy, propelling the body upwards and forwards. After toe-off at 309, the foot is positioned controlled through the swing phase until the next foot strike at 313.
For stair ascent depicted in FIG. 2, the human ankle-foot can be effectively modeled using a combination of an actuator and a variable stiffness mechanism. However, for stair descent, depicted in FIG. 3, a variable damper needs also to be included for modeling the ankle-foot complex; the power absorbed by the human ankle is much greater during stair descent than the power released by 2.3 to 11.2 J/kg {2}. Hence, it is reasonable to model the ankle as a combination of a variable-damper and spring for stair descent {2}.
Joint Biomechanics: The Human Knee
There are five distinct phases to knee operation throughout a level-ground walking cycle {8}. To further motivate the hybrid actuator design described herein, a description of these phases is included.                1. Beginning at the time the heel strikes as indicated at 403 in FIG. 4, the stance knee begins to flex slightly. This flexion period, called the Stance Flexion phase, allows for shock absorption upon impact as well as to keep the body's center of mass at a more constant vertical level throughout the stance period. During this phase, the knee acts as a spring, storing energy in preparation for the Stance Extension phase.        2. After maximum flexion is reached in the stance knee as indicated at 404, the joint begins to extend, until maximum extension is reached at 406. This knee extension period is called the Stance Extension phase. Throughout approximately the first 60% of Stance Extension, the knee acts as a spring, releasing the stored energy from the Stance Flexion phase of gait. This first release of energy corresponds to power output indicated at 501 in FIG. 5. During approximately the last 30% of Stance Extension, the knee absorbs energy in a second spring and then that energy is released during the next gait phase, or Pre-Swing, that begins at 406.        3. During late stance or Pre-Swing, the knee of the supporting leg begins its rapid flexion period in preparation for the swing phase. During early Pre-Swing, as the knee begins to flex in preparation for toe-off, the stored elastic energy from Stance Extension is released. This second release of energy corresponds to power output level indicated at 503 in FIG. 5.        4. As the hip is flexed, and the knee has reached a certain angle in Pre-Swing, the leg leaves the ground and the knee continues to flex as indicated at 407 in FIG. 4. At the time of toe-off at 407, the Swing Flexion phase of gait begins. Throughout this period, knee power is generally negative where the knee's torque impedes knee rotational velocity. During early Swing Flexion, the knee behaves as a variable damper, and during terminal Swing Flexion, the knee can be modeled as a spring, storing energy in preparation for early Swing Extension.        5. After reaching a maximum flexion angle during swing, the knee begins to extend forward as indicated at 408. During the early Swing Extension period, the spring energy stored during late Swing Flexion is then released, resulting in power output level indicated at 505 in FIG. 5. During the remainder of Swing Extension, the human knee outputs negative power (absorbing energy) to decelerate the swinging leg in preparation for the next stance period. After the knee has reached full extension, the foot once again is placed on the ground, and the next walking cycle begins at 410.        
Joint Biomechanics: The Human Hip
FIGS. 6 and 7 graphically depict human hip biomechanics for level ground walking Hip position (radians) is plotted in FIG. 6 and hip power (watts) is shown in FIG. 7, and show the spring-like behavior of the hip in walking Maximum hip power absorption occurs as indicated at 702 during terminal hip extension, and maximum hip power output occurs during active hip flexion near toe-off as indicated at 704 to drive the lower leg from the walking surface.
As discussed in more detail later, the hip can be modeled with a spring in parallel with a motor system. The parallel spring generally stores energy during hip extension and then releases that energy to power hip flexion. To the extent to which the desired joint behavior deviates from a conservative spring response, the hip model includes a parallel motor system designed to modulate stiffness, damping and power about the natural spring output.
Prior Art Leg Systems
The current state of the art in prosthetic leg systems include a knee joint that can vary its damping via magnetorheological fluid {5}, and a carbon fiber ankle which has no active control, but that can store energy in a spring structure for return at a later point in the gait cycle (e.g. the Flex-Foot or the Seattle-Lite) {4} {6}. None of these systems are able to add energy during the stride to help keep the body moving forward or to reduce impact losses at heel strike. In the case of legged robotic systems, the use of the Series Elastic Actuator (SEA) enables robotic joints to control their position and torque, such that energy may be added to the system as needed {7}. In addition, the SEA can emulate a physical spring or damper by applying torques based on the position or velocity of the joint. However, for most applications, the SEA requires a tremendous amount of electric power for its operation, resulting in a limited operational life or an overly large power supply. Robotic joint designs in general use purely active components and often do not conserve electrical power through the use of passive-elastic and variable-impedance devices.